A copper-clad laminate used as a base for printed circuit boards incorporated into various electronic devices is produced by hot-press bonding a copper foil having a sufficient thickness for a conductive circuit to an insulating carrier comprising, for example, a glass fiber/epoxy resin composite material.
One conventional process for producing a copper-clad laminate comprises electrodepositing copper on a rotating drum acting as a cathode, the surface of which has been polished to form an electrolytic copper foil of a prescribed thickness, continuously peeling the copper foil off the drum, after which the matte surface of the copper foil is hot-press bonded on an insulating carrier.
In recent years, a transfer process has been widely used, which comprises forming a copper deposit of a prescribed thickness on a conductive carrier having a smooth surface, e.g., a single plate of stainless steel, by electroplating, bringing the copper-plated side of the conductive carrier into contact with an insulating base, followed by hot-press bonding, and stripping the conductive carrier to transfer the copper deposit onto the surface of the insulating base.
In the above-described transfer process, copper is electrodeposited by, for example, electroplating, on the surface of a conductive carrier, such as a conductive metallic belt or single plate, to form a copper foil layer having a prescribed thickness. Then, a thin sheet of a prepreg is hot-press bonded to the surface of the copper foil layer to obtain an integral laminate, and the conductive carrier is peeled off from the laminate while transferring the copper foil layer to the prepreg. There is thus obtained a copper-clad laminate having a transferred copper foil layer.
With the recent demand for higher performance of printed circuit boards, high density mounting of various elements has been demanded. High-density printed circuit boards capable of forming a minute circuit pattern have also been studied.
To obtain such a high-density printed circuit board, it is advantageous to use an ultrathin copper-clad laminate having an extremely thin copper foil layer.
In general, a copper foil layer having pinholes has a poor appearance and a reduced commercial value. Various efforts have been made to avoid forming pinholes in the production of such an ultrathin copper-clad laminate by the transfer process.
While causes of pinholes in the transfer process are not always clear, it is considered that the surface condition of the conductive carrier used and the conditions of electroplating including plating bath composition, have great influence on pinhole formation.
Conductive carriers which can be used in the transfer process are generally prepared by rolling an ingot of stainless steel, nickel, or copper into a plate or a sheet. The surface of the resulting roller plate or sheet unavoidably contains metallurgical defects, such as pores produced during working (e.g., oil pits), non-metallic impurities or intermetallic compounds produced during melting. Since copper has a duplicating precision of about 0.05 .mu.m, if a copper foil layer is formed on such a conductive carrier, the surface defects are duplicated on the formed copper foil layer. When, in particular, there are undercut pits on the conductive carrier surface, build-up of copper is concentrated on the peripheral portion of the pits. When an insulating base is laminated on the thus-formed copper foil layer and then separated from the conductive carrier, the electrodeposited copper built up on a part of the undercut pits is torn off and remains on the carrier. As a result, the copper foil layer of the resulting copper-clad laminate has pinholes corresponding to the undercut pits.
Further, if a conductive carrier having a non-metallic impurity or an intermetallic compound exposed on the surface thereof is used, this produces a difference or a scatter in electrical conductivity on the surface of such a conductive carrier due to the difference in conductivity between the non-metallic impurity or intermetallic compound and the matrix metal. As a result, the degree of copper electrodeposition on the carrier varies according to site, and the resulting copper foil layer has pinholes.
Where the defects of the conductive carrier are physical, such as the above-described pores, it is possible to eliminate such defects by polishing the surface of the carrier. However, where a defect is a non-metallic impurity or an intermetallic compound contained in the carrier per se, it cannot be removed by surface polishing.
On the other hand, considering the pinhole problem from the aspect of electroplating conditions, it is known that when copper plating is conducted by using a plating bath comprising copper pyrophosphate or copper cyanide at a low current density, deposition of copper can be controlled by the action of a chelating agent added to the bath, and pinholes tend to be reduced. Nevertheless, such a method of copper electroplating achieves only a low rate of copper deposition and low productivity.
Thus, a copper foil layer formed by the conventional transfer processes typically exhibits a number of pinholes due to metallurgical defects on the surface of a conductive carrier used. Therefore, productivity and the non-defective yield of these processes are unavoidably reduced when applied to the production of ultrathin copper-clad laminates useful as a basic material of high-density printed circuit boards.
Further, a conductive carrier develops anisotropy on its surface during the preparation, i.e., rolling and surface treatment prior to copper foil formation, e.g., polishing. As a result, the copper foil layer formed on the anisotropic carrier surface also shows anisotropy in its mechanical characteristics, for example, a difference of elongation in crossing directions. Such anisotropy of a copper foil layer adversely affects the reliability of circuits through repetition of steps accompanied by heating for formation of a printed circuit board.
In the above-described transfer process, a copper foil layer, such as an electrolytic copper foil and a copper deposit, is usually subjected to the following surface treatments.
One is a surface roughening treatment for increasing adhesion strength to an insulating base onto which the copper foil layer is transferred. Such a treatment conventionally includes surface roughening by mechanical grinding or chemical grinding using an etching solution. Recently, a surface roughening method comprising precipitating ramiform projections on the surface of a copper layer surface by electrolytic copper plating to make the surface uneven is widely employed. On hot-press bonding to an insulating base, the ramiform projections are buried in the matrix resin of the insulating base to exert an anchoring effect, thus improving adhesion strength to the insulating base.
Another treatment, which follows the above-described surface roughening treatment, comprises forming a heat deterioration preventive layer on the roughened surface.
A copper foil layer of a copper-clad laminate is generally etched through a prescribed mask to form a conductive circuit corresponding to the mask pattern. Various IC chips are mounted on the conductive circuit by soldering. Therefore, the conductive circuit or adhesion areas between the circuit and the base at the soldered joints are partly heated at the time of soldering. As a result, the copper of the roughened surface undergoes heat deterioration and peels from the surface of the insulating base.
In this connection, a circuit board to be used in automobiles is required not to undergo heat deterioration in a heating tests of 180.degree. C. for 48 hours. Taking tests such as specification test into consideration, this requirement, though severe, is a reasonable heat deterioration property.
A heat deterioration preventive layer is a layer for preventing the above-mentioned heat deterioration and to assure adhesion between a copper foil layer and an insulating base, which usually comprises brass (Cu-Zn), an Ni-Cu alloy, an Sn-Cu alloy, or an Sn-Zn-Cu alloy.
The heat deterioration preventive layer is usually formed by electroplating a prescribed metal or alloy on the roughened surface of a copper foil layer or bonding a heat deterioration preventive layer on the roughened surface of a copper foil layer by using an adhesive.
Still another surface treatment is a rust preventive treatment. The purpose of this treatment is to prevent oxidative discoloration of the outside surface of a copper foil layer (i.e., the side opposite to an insulating base) during preservation and/or transportation thereby preventing a reduction in commercial value from the standpoint of appearance. The rust preventive treatment is generally conducted with a chromate. A treatment on only the outer side of a copper foil layer suffices for this purpose.
Thus, a copper-clad laminate composed of an insulating base having press-bonded thereon a copper foil layer has a structure such that the side of the copper foil layer in contact with the base has a roughened surface, and the roughened surface of the copper foil is coated with a heat deterioration preventive layer and, if desired, a rust preventive layer is further formed on the heat deterioration preventive layer.
A projection layer consisting of the roughened surface and the heat deterioration preventive layer formed thereon is generally called a profile layer. The profile layer has been a section formed by a serial combination of independent two steps of surface roughening of a copper foil layer and formation of a heat deterioration preventive layer.
In the profile layer, where the projections of the roughened surface are too high, that is, where the surface roughness of the copper foil layer is too large, the matrix resin of an insulating base press-bonded thereto does not reach the valleys of the projections, thus easily entrapping air bubbles, which causes insufficient adhesion between the copper foil layer and the base. The height of projections of the profile layer, i.e., surface roughness of the copper foil layer is usually from about 7 to 9 .mu.m, and the above-described unfavorable phenomenon often reduces the reliability of copper-clad laminates.
When a copper foil layer of a copper-clad laminate is subjected to a prescribed etching treatment, in cases where the projection height of the profile layer varies with place, that is, where the surface roughness of a copper foil layer is widely scattered, projections having a small height and being buried shallow in the matrix resin of an insulating base can be removed by etching, while projecting points of high projections are not etched and remain in the matrix resin. The non-etched residual copper (called stain) impairs electrical insulation between conductive circuit lines. Such a phenomenon is a serious problem, particularly in the formation of a fine circuit pattern having small distances between copper wires, sometimes impairing the function as a printed circuit board.
A countermeasure which has been taken to cope with the above-described stain problem is to reduce the total thickness of a copper foil layer. However, the conventional copper foil layer has the thickness of the main par (base copper) reduced, with the projection height in the profile layer or the scatter thereof being substantially unchanged. Therefore, formation of a fine pattern is still accompanied by the problem of copper remaining after etching.
As discussed above, there has not yet been developed a copper foil layer for copper-clad laminates, which has a profile layer having low projections, that is, small surface roughness, irrespective of the thickness of the base part of the copper foil, and a small scatter in surface roughness.